Recently, there is a growing demand for mobile electronic devices. Such mobile electronic devices typically include a secondary battery (i.e., a battery cell) as a power source. Among various secondary batteries, a lithium-ion secondary battery is the mainstream of the secondary batteries owing to its lightweight and high energy density. Since the secondary batteries deteriorate due to overcharge or overdischarge, they generally include protection circuits (or secondary battery protection circuits) to protect the secondary batteries from being overcharged or overdischarged. Lithium-ion secondary batteries are particularly susceptible to deterioration due to overcharge or overdischarge, and hence, it is preferable that lithium-ion secondary batteries be provided with the secondary battery protection circuits.
Among the mobile electronic devices, a notebook personal computer (or a notebook PC) includes a battery pack that includes parallel-connected modules, each of which includes serially-connected batteries. When the lithium-ion secondary battery is utilized in the battery pack, all the serially-connected batteries may need to be monitored by the protection circuits.
Japanese Patent Application Publication No. 2000-354335 (hereinafter referred to as “Patent Document 1”) discloses, for example, a technology for protecting serially-connected batteries from being overcharged or overdischarged, by providing protection circuits to respective batteries connected in series and connecting the protection circuits in parallel.
However, the technology disclosed in Patent Document 1 may need to include a photo-coupler or a field-effect transistor (FET) for integrating outputs of all the protection circuits of the batteries. Thus, a circuit structure may be complicated and hence, manufacturing cost may be increased.
Further, Japanese Patent No. 4080408 (hereinafter referred to as “Patent Document 2”) discloses, for example, a technology to provide protection IC for protecting batteries connected in series regardless of the number of the batteries connected in series.
In the technology disclosed in Patent Document 2, the serially-connected batteries are divided into blocks so that there are plural blocks each having plural serially-connected batteries, and protection ICs are provided for the respective blocks to monitor output voltage fluctuation in the blocks. This protection IC includes a detector circuit to monitor voltages of the batteries in the block, an output terminal to externally output signals received from the detector circuit, a connecting terminal to connect the protection IC to a different protection IC, and an output circuit connected between the output terminal and the connecting terminal. Further, this output circuit includes a current source, a transistor, a changing device to change an electrical condition between the output terminal and the connecting terminal based on an output result from the detector circuit, and a transmitting device to transmit signals supplied to the connecting terminal to the output terminal via the different protection IC. In the output circuit, the current source is switched ON or OFF based on the output result of the detector circuit, and the current output from the current source is directly transmitted to the output terminal. Further, one of a source and a drain of the transistor is connected to the output terminal and the other connected to the connecting terminal to provide cascade connection between the detected result output terminals in each of the protection ICs. With this configuration, it may be possible to detect overcharge or overdischarge of all the blocks of batteries based on an output result of a final terminal of the protection IC.
However, in the technology disclosed in Patent Document 2, for example, if two serially-connected battery cells are protected by two protection circuits, a Cout/Dout terminal (i.e., a charge/discharge control signal transmitting terminal) of one of the battery cells transmits a signal to a CTLC/CTLD terminal (i.e., a charge/discharge control signal receiving terminal) of the other battery cell. At this moment, the amount of voltage applied to the internal elements may correspond to a sum of the voltages of the two battery cells at the maximum. Thus, the internal elements may need to be formed of high voltage resistant elements in order to withstand such amount of voltage. This may result in an increase in a process cost or increase in a layout area.
Japanese Patent Application Publication No. 2009-17732 (hereinafter referred to as “Patent Document 3”) and Japanese Patent Application Publication No. 2009-195100 (hereinafter referred to as “Patent Document 4”) disclose, for example, a technologies to withstand a large amount of voltage. However, current may flow from the Cout/Dout terminal of one of the battery cells to the CTLC/CTLD terminal of the other battery cell despite capabilities of the internal elements to withstand the large amount of voltage.
Thus, it may be necessary to provide an external voltage resistant element or a high voltage resistant element inside the IC to lower the current flowing from the Cout/Dout terminal of one of the battery cells to the CTLC/CTLD terminal of the other battery cell. However, providing the external voltage resistant element may result in an increase in component cost and an increase in a mounting area, and further, providing the high voltage resistant element inside the IC may result in an increase in the layout area.
Accordingly, it may be desirable to provide protection circuits that may protect battery cells without having the high voltage resistant element and without causing the current to flow between the protection circuits.
More specifically, it may be desirable to provide a secondary battery protection circuit that may suppress increases in the process cost, the layout area, the component cost, or the mounting area of the secondary battery protection circuit.